In general, a compressor is a mechanical apparatus for compressing the air, refrigerant or other various operation gases and raising a pressure thereof, by receiving power from a power generation apparatus such as an electric motor or turbine. The compressor has been widely used for an electric home appliance such as a refrigerator and an air conditioner, or in the whole industry.
The compressors are roughly classified into a reciprocating compressor in which a compression space for sucking or discharging an operation gas is formed between a piston and a cylinder, and the piston is linearly reciprocated inside the cylinder, for compressing a refrigerant, a rotary compressor in which a compression space for sucking or discharging an operation gas is formed between an eccentrically-rotated roller and a cylinder, and the roller is eccentrically rotated along the inner wall of the cylinder, for compressing a refrigerant, and a scroll compressor in which a compression space for sucking or discharging an operation gas is formed between an orbiting scroll and a fixed scroll, and the orbiting scroll is rotated along the fixed scroll, for compressing a refrigerant.
Recently, a linear compressor which can improve compression efficiency and simplify the whole structure without a mechanical loss resulting from motion conversion by connecting a piston directly to a linearly-reciprocated driving motor has been popularly developed among the reciprocating compressors.
FIG. 1 is a view for conceptually explaining a conventional reciprocating compressor. A cylinder 4 required for compression and expansion and a piston 6 provided inside the cylinder and capable of reciprocating are included. The cylinder 4 includes a discharge valve 24 which is opened and closed according to a pressure of a compression space in the cylinder 4. The reciprocating compressor includes a motor 10 for providing power required for compression. The motor comprises an inner stator 12 formed by laminating a plurality of laminations in the circumferential direction, and installed outside the cylinder 4, an outer stator 14 having a coil winding body 14a formed therein, and installed outside the cylinder 4 with a predetermined gap from the inner stator 12, and a permanent magnet 16 positioned at the gap between the inner stator 12 and the outer stator 14, and connected to the piston 6. A back cover 2 connected to one surface of the piston 6 and the cylinder 4 is connected by a spring 5, preferably, a coil spring.
In the reciprocating motor 10, when a current is applied to the coil winding body 14a to generate an electromagnetic force, the permanent magnet 16 is linearly reciprocated by interactions between the electromagnetic force and the permanent magnet 16, and the piston 6 connected to the permanent magnet 16 is linearly reciprocated inside the cylinder 4.
FIG. 2 is a view illustrating a circuit diagram for applying power to the motor of the conventional reciprocating compressor. There is included a DC power supply unit 22 for rectifying an AC power (not shown) for home use or industrial use and converting it into a DC, which is controlled by inverter switches S1 to S4. Although a rectifier section for rectifying an AC power and a DC link section for stabilizing the rectified power are included, they are omitted.
The DC power controlled through the rectifier section (not shown) and the DC link section (not shown) is converted into an AC power having an appropriate amplitude and frequency through the inverter switches S1 to S4, and the AC power Vm is applied to the motor (10 of FIG. 1). However, such a power supply apparatus employs no less than four switches, and hence the price is not cheap and the power supply apparatus becomes complicated.
FIG. 3 is a graph showing the operation of the conventional reciprocating compressor and a view showing a modeling of the reciprocating compressor.
When no power is applied to the motor (10 of FIG. 1) and there is no external force applied thereto, the distance between the head of the piston 6 and one surface of the cylinder constituting the compression space (hereinafter, the initial value) is denoted by Xi. A point at which the head of the piston 6 is the closest to one surface of the cylinder constituting the compression space is referred to as a top dead center or top dead point, and a point at which the head of the piston 6 is the farthest from one surface of the cylinder constituting the compression space is referred to as a bottom dead center or bottom dead point. The piston 6 is provided inside the cylinder 4 so as to be reciprocatable. The mass of the piston is m, the elastic coefficient of the spring for connecting the piston 6 to the back cover (2 of FIG. 1) connected to the cylinder is k, and a resistant constant is Cf. ΔP is the difference between a discharge pressure and suction pressure in the cooling cycle of a cooling apparatus. ΔP·As is a force shifting the piston 6 due to the difference between the discharge pressure and suction pressure, which can be modeled as a gas spring. For efficiency purpose, the top dead center is preferably one surface of the cylinder 4. That is, it is preferred that Xi=0 at the top dead center. When no power is applied to the motor 10 and there is no other external force applied thereto, the piston 6 is located at the middle between the top dead center and the bottom dead center.
The operation for one cycle of the conventional reciprocating compressor is as follows. If it is assumed that the piston 6 starts from C, Xi is the largest and the discharge valve (not shown) is in a closed state. When the piston 6 moves and Xi is decreased, the pressure rises up to a level enough to open the discharge valve (D). When the discharge valve (not shown) is opened, the compressed fluid (refrigerant) is discharged, there is no change in pressure, and one surface of the piston is moved until Xi=0(A). Upon completion of compression to the top dead center, the piston 6 is moved in the opposite direction. If Xi increases, the volume of the compression space becomes larger, and Xi increases to more than a predetermined value, the discharge valve is closed. If the suction process is performed in a state the discharge valve is closed, Xi increases, and thus the pressure decreases and returns to its original position C, thereby completing one cycle.
In such a reciprocating compressor, the power supply unit (power supply apparatus) for supplying power to the motor employs no less than four switches, and hence the price is not cheap and the power supply apparatus is complicated and not easy to control.